Address warnings in vignettes

[marohrdanz]

Vignettes that include htmltools::includeHTML() now show warnings such as the following on our documentation website:

library('htmltools')
filePath = paste(getwd(),'/tcga-brca.html',sep='')
htmltools::includeHTML(filePath)
#> Warning: `includeHTML()` was provided a `path` that appears to be a complete HTML document.
#> ✖ Path: /Users/runner/work/NGCHM-R/NGCHM-R/vignettes/tcga-brca.html
#> ℹ Use `tags$iframe()` to include an HTML document. You can either ensure `path` is accessible in your app or document (see e.g. `shiny::addResourcePath()`) and pass the relative path to the `src` argument. Or you can read the contents of `path` and pass the contents to `srcdoc`.

Address these so the either go away, or are not displayed on the rendered vignette.

Affected vignette pages:

• https://md-anderson-bioinformatics.github.io/NGCHM-R/articles/create-a-ng-chm.html
• https://md-anderson-bioinformatics.github.io/NGCHM-R/articles/covariate-bars-and-discrete-color-maps.html
• https://md-anderson-bioinformatics.github.io/NGCHM-R/articles/continuous-color-maps-and-data-layers.html

Note: vignettes are NOT uploaded to CRAN. So if desired, we can fix this w/o making a new CRAN release.